One time when speaking to Rav Schach, someone used the common expression, that a posek or rabbi must use the “fifth volume of Shulchan Aruch.” This phrase is supposed to convey the idea that although the S”A only has 4 volumes, one also must apply common sense and logical insight — “the 5th volume” — when issuing a halachic ruling in cases that are not clear cut in the “other” 4 volumes.
Rav Schach objected to the phrase. “Although the idea that underlies the saying is true, one must be extremely careful with his choice of words. We students of the Torah know that this is only a clever idiomatic expression, and that the S”A only has 4 sections. But today, when there are so many people who bend and corrupt the halacha to suit their own individual versions of Judaism, it is not worthwhile to this expression at all, for baalei battim are liable to understand from that there is a license to add or to adopt the rulings of the S”A to one’s own liking. They are likely to form the impression that whenever a dayan or rabbi issues any ruling at all it is all based on his subjective self-devised ‘fifth volume’, and therefore not binding. You see, as a result of the careless use of a witty expression, it is possible to impart a false notion to many people, and it is better to avoid any expression that bears the possibility of having a negative impact.”